The present invention relates to establishing a reference pressure level for use in triggering the operation of a medical ventilator supplying breathing gases to a patient. Through use of the invention, the operation of the ventilator in supporting spontaneous breathing efforts by the patient may be enhanced.
Patients in need of respiratory assistance are often placed on a mechanical ventilator. A mechanical ventilator is a device that provides breathing gas to a patient who is unable to breath, or alternatively, detects a patient's attempt to breathe and provides breathing gases to assist in that effort. The breathing gases typically comprise air, or air mixed with other gases, such as additional oxygen. The breathing gases are delivered to the patient from the ventilator via flexible tubes comprising the patient breathing circuit.
The patient breathing circuit comprises an inspiratory limb which supplies the breathing gases to the patient from the ventilator and an expiratory limb which directs the expired gases away from the patient. Gas flow is facilitated by a Y-connector for the inspiratory limb and expiratory limb and for a patient limb or connector leading from the Y-connector to the patient. A plurality of one way check valves in the breathing circuit direct gas flows in the breathing circuit. A patient interface, such as a face mask or endotracheal tube, administers the breathing gases from the breathing circuit to the patient. Flow sensors and pressure transducers are placed in the breathing circuit to monitor the delivery of breathing gases for safety and control purposes.
In some situations, such as those found in critical care units, emergency rooms, or operating rooms, a patient may be spontaneously breathing yet be too weak or sedated to carry out sufficient respiratory activity by himself/herself. Or, the respiratory system of the patient may be injured or too inefficient for adequate respiration. In these instances, the patient may start an inspiration of breathing gases but is not able to overcome the resistance to the flow of breathing gases in the breathing circuit to continue a full respiratory cycle. The mechanical ventilator is then used to assist the patient in breathing. This assistance is provided by delivering breathing gases to fill the lungs with breathing gases. At the end of the mechanical assistance, the supply of breathing gas to the patient terminates and the natural compliance of the patient's chest wall forces the breathing gases out of the lungs in an expiration, thus completing a mechanically assisted respiratory cycle.
An analogous situation occurs when weaning a patient receiving mechanical ventilation off the ventilator to a state of spontaneous breathing.
When assisting the breathing of a patient, the mechanical ventilator delivers breathing gases to the patient when a patient breathing attempt is detected. This detection is typically accomplished by using sensors in the breathing circuit to detect changes in conditions in the breathing circuit as the patient attempts to inhale. It is desirable to make the initiation, or “triggering” of ventilation sensitive, or responsive, to the breathing attempts of the patient as a patient being mechanically ventilated is often in a weakened condition and the patient's spontaneous breathing attempts while requiring a relatively large amount of inspiratory work on the part of the patient may not produce much of a change in conditions in the breathing circuit.
In the past, flow sensing has been generally recognized as providing a more sensitive triggering action. This is due to the fact a pressure level to serve as a reference for a pressure triggering action has been difficult to establish with the necessary degree of accuracy.